When I try to make a 3 way call on SP1 of my Obi202, after I connect the first party, hit the flash button on my phone, and try to dial the 2nd party, I get a recording saying "No service configured error ..."

Do I have to make changes in my Obi expert config to get 3 way calling to work? My phone service, Voip MS, says that feature is supported with my line.

I previously had Call Waiting turned off and I think that's why in the Expert Config screen, it said MaxSessions was set to 1. I changed it to 2, saved the changes, and tested it. But 3-way calling still does not work.

Anything else I should change in the settings to get 3 way calling to work?

You don't need to define anything special for 3-ways. Your service provider needs to support multiple simultaneous calls and MaxSessions needs to be 2 or greater. Disabling CallWaiting shouldn't change MaxSessions. Make sure you are changing the correct MaxSessions. If your service provider is defined on SP1 then:Service Providers -> SP1 Service -> MaxSessions: 2

Just in case I don't understand 3-ways you can try making simultaneous calls without a 3-way. Make a call from Phone1. While the 1st call is active, make a 2nd call from Phone2. If this fails, it will prove the failure is not related to 3-ways.